Pattern Syntax in Rust
All valid syntax to use with pattern matching.
Matching Literals
This pattern is useful when we want our code to take action when it receives a concrete value.
fn main() {
let x = 1;
match x {
1 => println!("one"),
2 => println!("two"),
3 => println!("three"),
_ => println!("anything"),
}
}
Matching Named Variables
Second branch is using named variable pattern to match other variable shadowing any variable outside of match block.
fn main() {
let x = Some(5);
let y = 10;
match x {
Some(50) => println!("Got 50"),
Some(y) => println!("Matched, y = {:?}", y),
_ => println!("Default case, x = {:?}", x).
}
}
Multiple patterns
fn main() {
let x = 1;
match x {
1 | 2 => println!("one or two"), // using OR operator |
2 => println!("three"),
3 | 4 | 5 => println!("greater than 2 or equal to 5"),
_ => println!("anything"),
}
}
Matching Ranges of Values
The range operator only works on numeric values and characters.
fn main() {
let x = 5;
match x {
1..=5 => println!("one through five"),
_ => println!("something else"),
}
let x = 'c';
match x {
'a'..='j' => println!("early ASCII letter"),
'k'..='z' => println!("late ASCII letter"),
_ => println!("something else"),
}
}
Destructuring to Break Apart Values
Destructuring structs
We deconstruct p a Point instance to create a and b.
a is going to be mapped to whatever values contained in x and b to y.
struct Point {
x: i32,
y: i32,
}
fn main() {
let p = Point { x: 0, y: 7};
let point { x: a, y: b} = p;
assert_eq!(0, a);
assert_eq!(7, b);
}
However it’s common to have variables inside of a pattern match the field names, so:
let point { x, y } = p;
assert_eq!(0, x);
assert_eq!(7, y);
When destructuring a struct we can use named variables and literals.
fn main() {
let p = Point { x: 0, y: 7};
match p {
Point { x, y: 0 } => { // `y` must be zero
println!("On the x axis at {}", x)
},
Point { x: 0, y} => { // `x` must be zero
println!("On the y axis at {}", y)
},
Point { x, y } => {
println!("On neither axis: ({}, {})", x, y)
}
}
}
Destructuring Enums
enum Message {
Quit, // unit like variant
Move { x: i32, y: i32}, // struct variant
Write(String), // tuple variant called `Write`
ChangeColor(i32, i32, i32), // tuple variant with 3 values called `ChangeColor`
}
fn main() {
let msg = Message::ChangeColor(0, 160, 255);
match msg {
Message::Quit => {
// a semicolon isn't required if there is only one expression
println!("Quit");
}
Message::Move { x, y } => {
println!("Move to x: {} y: {}", x, y)
}
Message::Write(text) => {
println!("Text message: {}", text)
}
Message::ChangeColor(r, g, b) => {
println!("Change color: red {}, green {}, and blue {}", r, g, b)
}
}
}
Destructuring nested structs and Enums
enum Color {
Rgb(i32, i32, i32),
Hsv(i32, i32, i32),
}
enum Message {
Quit, // unit like variant
Move { x: i32, y: i32}, // struct variant
Write(String), // tuple variant called `Write`
ChangeColor(Color), // tuple variant with 3 values called `ChangeColor`
}
fn main() {
let msg = Message::ChangeColor(
Color::Hsv(0, 160, 255)
);
match msg {
Message::ChangeColor(Color::Rgb(r, g, b)) => {
println!("Change color: red {}, green {}, and blue {}", r, g, b)
}
Message::ChangeColor(Color::Hsv(h, s, v)) => {
println!("Change color: hue {}, saturation {}, and value {}", h, s, v)
}
}
_ => (),
}
struct Point {
x: i32,
y: i32,
}
fn main() {
// destruct 3 and 10 into `feet` and `inches`
let ((feet, inches), Point { x, y }) = ((3, 10), Point { x: 3, y: -10 });
}
Ignoring Values in a Pattern
_ can be used anywhere to signify ingoring of the value.
This could be useful when we need a function signature to be something specific but we’re not gonna use all the arguments passed in. The _ avoid compiler warning for unused parameters.
fn main() {
foo(3, y);
}
fn foo(_, i32, y: i32) {
println!("This code only uses the y parameter: {}", y);
}
To ignore part of a value
If the setting has no value then we can set a value, however if the setting already has a value then we can’t modify it.
fn main() {
let mut setting_value = Some(5);
let new_setting_value = Some(10);
match (setting_value, new_setting_value) {
// if both are `Some<T>` variant then
(Some(_), Some(_)) => {
println!("Can't overwrite an existing customized value")
}
_ => {
setting_value = new_setting_value
}
}
println!("setting is {:?}", setting_value);
}
To ignore values at multiple places withing one pattern
fn main() {
let numbers = (2, 4, 8, 16, 32);
match numbers {
(first, _, third, _, fifth) => {
println!("Some numbers: {}, {}, {}", first, third, fifth)
}
}
}
Unused variables
The compiler won’t complain.
fn main() {
let _x = 5;
let y = 10;
}
Prefixing a name with _ is different than just using _.
Prefixing a variable name with _ still binds the value.
_s is still binding with s.
fn main() {
let s = Some(String::from("Hello!"));
// to avoid compiler complain from not using `s` inside we prefix it with `_`
// but this moves value to `_s`
if let Some(_s) = s {
println!("found a string")
}
println!("{:?}", s);
// ^ error: moved value
}
Instead if we would have done this, then it would successfully compile:
fn main() {
let s = Some(String::from("Hello!"));
if let Some(_) = s {
println!("found a string")
}
println!("{:?}", s);
}
Range syntax to ignore values
fn main() {
struct Point {
x: i32,
y: i32,
}
let origin = Point { x: 0, y:0, z: 0 };
match origin {
// ignore all field except `x`
Point { x, .. } => println!("x is {}", x),
}
}
fn main() {
let numbers = (2, 4, 8, 16, 32);
match numbers {
(first, ..., last) => {
println!("Some numbers: {}, {}", first, last);
}
}
}
But they should be unambigous. Something like this won’t work:
(..., second, ...) => { }
Match Guards
A match guard is an additional if condition specified after the pattern in a match arm that must also match along with the pattern.
Useful for expressing complex ideas that patterns themselves cannot express.
fn main() {
let num = Some(4);
match num {
Some(x) if x < 5 => println!("less than five: {}", x),
// ^^^^^^^^ a match guard
Some(x) => println!("{}", x),
None => (),
}
}
Match guards also solve issues with shadowing inside the match block.
Here in this example, we want to execute some code when x is equal to y.
We can’t use y inside the pattern because it will shadow the outside y. But we could use y inside of our match guard.
fn main() {
let x = Some(5);
let y = 10;
match num {
// match on any `Some<T>`; bind it to `n` and compare it to `y`.
Some(n) if n == y => println!("Matched: x = {}", n),
_ => println!("Default case, x = {:?}", x),
}
}
Multiple patterns with match guards:
fn main() {
let x = 4;
let y = false;
match x {
// `x` has to either 4 or 5 or 6 AND `y` has to be true
4 | 5 | 6 if y => println!("yes"),
_ => println!("no"),
}
}
Bindings
@ operator let’s us create a variable that holds a value at the same time we’re testing that value to see whether it matches a pattern.
fn main() {
enum Message {
Hello { id: i32 },
}
let msg = Message::Hello { id: 5 };
match msg {
Message::Hello {
// match `id` between 3 and 7 and also store it in `id_variable`
// can be same as field name; instead of `id_variable`
id: id_variable @ 3..=7,
} => println!("Found and id in range: {}", id_variable),
Message::Hello { id: 10..=12 } => { // otherwise simpy use range operator
println!("Found an id in another range")
}
Message::Hello { id } => { // to bind the variable
println!("Found some other id: {}", id)
}
}
}